Tonight marks the first Republican presidential debate since Donald Trump dominated Super Tuesday, as the “Stop Trump” effort is now in full force.

Only four candidates are slated to take the stage for the faceoff: Donald Trump, Sen. Marco Rubio of Florida, Sen. Ted Cruz of Texas and Ohio Gov. John Kasich. (Retired neurosurgeon Ben Carson remains in the race, but announced Wednesday he sees no “political path forward” and has decided not to attend).

Tonight’s debate is hosted by Fox News and set to begin at 9 p.m. E.T. in Detroit. Here are the five things to watch for in tonight’s GOP showdown:

1. Trump vs. Kelly Rematch

Donald Trump will go toe-to-toe with debate moderator Megyn Kelly for the first time since the candidate got into a feud with the Fox News host after she asked him about his previous comments about women in the first Republican debate.

Before the Iowa caucuses, Trump skipped the next Fox-hosted debate and held an event to benefit veterans groups instead, after a back-and-forth battle of statements and attacks.

2. Responding to Romney

Though he won’t be on the debate stage, expect Mitt Romney to be a topic of conversation.

The 2012 GOP nominee’s comments today on the state of the 2016 race -- specifically bashing Trump as a “phony” and a “fraud” -- or his comments on Trump’s taxes will most likely generate questions.

But the biggest bombshell that all the candidates will likely be asked about is Romney’s apparent call for a contested convention:

“Given the current delegate selection process, this means that I would vote for Marco Rubio in Florida, for John Kasich in Ohio, and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state,” he said.

3. Which Rubio to Expect?

At the most recent Republican debate, Rubio went after Trump vigorously, unloading the opponent playbook on Trump.

On the campaign trail lately, Rubio has doubled down, making fun of Trump’s sweat, “spray tan,” and suggesting Trump wet his pants during the debate.

After a disappointing Super Tuesday showing, however, the open question is whether Rubio will change his game plan at tonight’s debate or keep up the attacks.

4. Cruz Plays Defense

After Tuesday, Trump congratulated Cruz on his major win.

"This has been an amazing evening. I want to congratulate [Cruz] on winning Texas, that was an excellent win," Trump said at his Mar-a-Lago Club in Palm Beach, Florida.

Cruz, however, called on the rest of his fellow candidates to unify behind him and defeat Trump.

“For the candidates who have not yet won a state, who have not racked up significant delegates, I ask you to prayerfully consider our coming together, uniting,” Cruz said. “That is the only way to beat Donald Trump. Our campaign beats Donald Trump resoundingly. For that to happen we must come together.”

At the most recent Republican debate, Cruz and Rubio went after Trump, and after Tuesday’s victories, it will be up to Cruz: Will he take on Trump or steer clear?

5. Kasich Clings On

Kasich’s Super Tuesday showing was anything but “super,” and he has yet to win one state, and even so the governor said he was proud to have “exceeded expectations.”

Kasich’s being faced with the “why are you even here” question would come as no surprise, especially after Ben Carson decided not to participate in tonight’s debate.

But, the Ohio governor says, he is confident his campaign will move forward as he gets into “home court” advantage.